1. Field of the Invention
The present invention relates to communications, and, in particular, to cellular communications systems.
2. Description of the Related Art
The IS-95 standard, an interim standard published by the Telecommunications Industry Association, is an existing cellular communications standard that is based on code division multiple access (CDMA) methodology. In CDMA methodology, different channels are distinguished by different codes, where the signals for each channel are spread over the entire available frequency band. This differs from traditional analog schemes in which each channel is designated a unique portion of the available frequency band.
Referring now to FIG. 1, there is shown a block diagram of the forward link of a cellular network 100 conforming to the IS-95 standard. The forward link in a communications network is the communication path from the base station to a user (e.g., a cellular telephone). The reverse link, on the other hand, is the communication path from the user back to the base station. Network 100 comprises M IS-95 signal generators 102 (where M is an integer greater than 0), combiner 104, and radio frequency (RF) circuitry 106 including antenna 108. Each signal generator 102 receives narrowband data streams for up to 64 different users and processes that narrowband data to generate a communications signal conforming to the IS-95 standard. Each signal generator 102 in network 100 generates an IS-95 signal at a different carrier frequency.
According to the IS-95 standard, the narrowband data stream for each user is multiplied by a particular code sequence and then modulated at a particular carrier frequency. For a given signal generator, the narrowband data stream for each user is encoded with a different code sequence, but modulated at the same carrier frequency. The effect of modulating the narrowband data for multiple users at the same carrier frequency is to spread all of the narrowband data for each user over the entire carrier-frequency band. In order to ensure that the modulated signals for different users do not interfere with one another, the code sequences are selected to ensure that the modulated signal for each user is orthogonal to the modulated signals for all other users in the same carrier-frequency band.
The signals from the different signal generators are combined by combiner 104. The combined signal is processed by RF circuitry 106 for transmission by antenna 108 to any number of remote cellular units (e.g., mobile telephones) (not shown).
Referring now to FIG. 2, there is shown a block diagram of a portion of each signal generator 102 of cellular network 100 of FIG. 1. Under the IS-95 standard, each signal generator 102 is capable of supporting narrowband data streams from up to 64 different users usinig a single carrier frequency. Each user is assigned a different one of 64 orthogonal IS-95 forward-link Walsh codes, also known as Walsh functions or Walsh sequences. FIG. 2 shows the processing performed on the data stream for one of the users supported by signal generator 102. That is, the block diagram shown in FIG. 2 would be repeated within signal generator 102 for each user with its own narrowband data.
In particular, for a particular user, convolutional encoder 202 provides a degree of error protection by applying convolutional encoding to the user's narrowband data to generate encoded signals. Block interleaver 204 applies block interleaving to the encoded signals to generate interleaved signals. Those skilled in the art will understand that block interleaver 204 provides further error protection by scrambling data in time. In a parallel path, long pseudo-noise (PN) code generator 206 generates code signals that are then decimated by decimator 208. PN code generator 206 and decimator 208 perform encryption to provide a degree of security to the communications process. The interleaved signals from bloc interleaver 204 are combined with the decimated code signals from decimator 208 by multiplier 210.
The resulting signals from multiplier 210 are combined with one of the 64 different Walsh sequences W.sub.N by Walsh-code multiplier 212. Those skilled in the art will understand that multiplying signals by a unique Walsh sequence W.sub.N makes the resulting signals orthogonal to (and therefore non-interfering with) the signals for all of the other users of signal generator 102, each of which is multiplied by a different Walsh sequence.
The signals generated by Walsh-code multiplier 212 are then processed along two parallel paths. In the first path, multiplier 214 combines the signals from Walsh-code multiplier 212 with the signal P.sub.I (t) and the signals from multiplier 214 are then combined by multiplier 216 with the signals (cos w.sub.cm t), where w.sub.cm is the carrier frequency for the m.sup.th signal generator 102 of network 100. In the second path, multiplier 218 combines the signals from Walsh-code multiplier 212 with the signal P.sub.Q (t) and the signals from multiplier 218 are then combined by multiplier 220 with the signals (sin w.sub.cm t). P.sub.I (t) and P.sub.Q (t) are the in-phase part and the quadri-phase part, respectively, of short PN codes used in quadrature-phase shift-keying (QPSK) spread-spectrum modulation. As such, multipliers 214 and 218 further whiten the output to ensure that the signals are spread over the full carrier-frequency band. Multipliers 216 and 220 modulate the signals by the carrier frequency w.sub.cm.
The signals from multipliers 216 and 220 are then combined at summation node 222 to generate one of up to 64 different output signals transmitted from each IS-95 signal generator 102 to combiner 104 of FIG. 1. Those skilled in the art will understand that multipliers 214-220 and summation node 222 combine to operate as a signal modulator/spreader.
Networks conforming to the IS-95 standard are limited to 64 users for each carrier frequency. Moreover, each user is limited to relatively low data-rate communications such as telephone-based voice signals. Under the IS-95 standard, each data stream is limited to a maximum of 9600 bits per second (bps). Thus, while IS-95 networks are sufficient for typical use by multiple mobile telephone users, they are nevertheless unable to support high data-rate applications. What is needed therefore is a cellular communications system that supports data-rate applications higher than those supported by conventional IS-95 networks. Since the equipment for such communications networks is extremely expensive and since an IS-95 infrastructure already exists, it is also desirable to provide a solution that is backwards compatible with IS-95 technology and the existing IS-95 infrastructure.
Further objects and advantages of this invention will become apparent from the detailed description which follows.